This invention relates generally to aircraft aerodynamics and specifically to a method and apparatus for extracting energy from the lift-induced vortex at or near the wingtips of aircraft.
Under lifting conditions, an airfoil creates a vortex system which is shed along the span, with a concentrated vortex centered just behind each wingtip. This vortex is a pure by-product of lift. It is not necessary for the production of lift and is a major source of aircraft induced drag. The vortex system generated by large aircraft (above 250,000 lbs.) poses serious hazards for following aircraft. Smaller aircraft in established flight paths are affected seriously by such vortex systems.
Much research has concentrated on attenuation of this lift-induced vortex. Commonly known means for dissipation or attenuation of the vortex include vertical winglets attached at the wingtips, such as the plates described in U.S. Pat. No. 3,411,738 to Sargent. Various fan-like attachments designed to break up the vortical airflow at the wingtips, to reduced hazards to following aircraft, have also been tested. U.S. Pat. Nos. 3,984,070 and 3,934,844, to Patterson, Jr. and Reighart II, respectively, are representative of this type of art. The patent to Patterson, Jr. discloses a vortex-attenuating spline device attached at the wingtips. That to Reighart II describes a vortex generator which creates a vortex opposite in direction to the wingtip vortex, in effect cancelling some of its effects.
These prior art attachments yield energy in a sense, in the form of decreased drag, but no means has been shown to extract from the wingtip vortex energy directly usable in an aircraft.
More recently, some research has focused on the use of blades oriented transversely in the airstream vortex to dissipate vortical energy. Fixed, cambered vanes have been placed one chord length behind the airfoil trailing edge. These vanes provide induced drag reduction, but do not allow transfer of energy for use elsewhere in the aircraft.
A tip turbine has been tested for its "energy extraction" potential. Net power recovery in the form of decreased drag is optimum at turbine rotation=0 rpm; study discloses that a nonrotating turbine placed in the wingtip area will replace a single concentrated vortex with a series of vortices emanating from the vane tips. The "energy extraction" of the nonrotating turbine, however, is not available for utilization as mechanical energy.
In summary, prior art shows no means for extracting energy from the lift-induced vortex, for further utilization within the aircraft. It is an object of the present invention to provide an apparatus capable of extracting energy from the lift-induced vortex and converting it into a form of energy directly usable within an aircraft.
It is another object of the invention to provide such an apparatus wherein the vortex energy is attenuated.
It is another object of the invention to provide such an apparatus wherein the lift-induced vortex energy may be extracted and used as rotational energy.
It is another object of the invention, to provide such an apparatus wherein turbine blades intercept the vortex airflow behind the trailing edge of the aircraft wing.
It is still another object of the invention to provide such an apparatus wherein rotation of turbine blades powers a generator, pump, or similar apparatus within the aircraft.
It is yet another object of the invention to provide such an apparatus wherein the turbine blades are tapered and cambered for maximum efficiency.
It is a further object of the invention to provide such an apparatus wherein the decrease in induced drag reduction associated with rotation of the turbine blades is more than offset by the energy transmitted for utilization within the aircraft.
It is another object of the invention to provide such an apparatus wherein the airfoil-turbine blade angle of attack is such that energy transmitted for further utilization is high in proportion to the decrease in induced drag reduction brought about by rotation of the turbine blades.
It is another object of the invention to provide such an apparatus wherein the energy transmitted to the shaft may be utilized for airfoil boundary layer control or similar use.